Increased sensitivity and accumulation of estradiol in the rat oviduct during early pregnancy

We have previously reported that a single injection of estradiol-17β (E₂) given on day 3 of pregnancy (P3) is far more effective for accelerating oviductal transport in the rat, than treatment given on day 1 (P1). In order to quantify this change, dose-response curves were established for six different doses of E₂ (range 0.031 to 1.00 μg per animal) given on P1, P2 or P3. In addition, a possible mechanism was explored by comparing the plasmatic and oviductal levels of E₂ between 30 and 180 min following treatment with E₂ on P1 or P3. As the interval from ovulation to treatment was increased, the transport of a larger number of embryos was accelerated and a smaller dose was required. The minimal effective dose decreased 30-fold from P1 to P3, the oviducts accumulated 20% to 90% more E₂ on P3 than on P1, tissue levels were 6- to 48-fold higher than plasma levels and the latter did not differ between P1 and P3. It is concluded that the oviduct exhibits increased sensitivity and responsiveness to E₂ on P3 and this is associated with greater accumulation of the hormone in the organ, not attributable to higher E₂ plasma levels.